1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus which can set appropriate tube currents (mA) for different regions upon one helical scan by designating an image SD (Standard Deviation) for each region.
2. Description of the Related Art
As is known, an X-ray computed tomography apparatus is designed to obtain an image (tomographic image) by calculating (reconstructing) the X-ray absorption coefficient of tissue such as an organ on the basis of the amount of X-rays absorbed in a subject to be examined as an index called a CT value with reference to the X-ray absorption coefficient of water.
A reconstructed image inevitably contains image noise. Image noise is typically defined with reference to a variation in the CT value of a homogeneous phantom image as a standard deviation, which is generally referred to as an image SD. In order to make diagnosis by observing a reconstructed image, e.g., to discriminate a shadow on the image as noise or a tumor, the image SD unique to the image must be considered.
FIG. 1 is a view for explaining a technique called conventional real EC. Conventional real EC is used to automatically calculate a tube current (mA) for a designated image SD from the CT value of a scanogram at a corresponding position. This technique determines a tube current so as to obtain the image SD designated at each position. Therefore, images with uniform image quality can be acquired regardless of the physiques of patients. In addition, the exposure to X-rays of a subject can be reduced as compared with a case wherein data is acquired with a constant tube current.
There is a strong tendency that such an image SD depends on the transmission dose of X-rays which is mainly determined by the relationship between a tube current and a subject to be examined. For this reason, the image SD for obtaining clinically required image quality varies depending on an imaging region, a tumor as a diagnosis target, or the like. In consideration of such a general situation, an image SD is preferably set for each imaging region in a sequence in terms of a reduction in the exposure to X-rays.
In a conventional system, however, an image SD cannot be set for each imaging region in a sequence. For this reason, when an image SD is to be changed for each imaging region so as to further reduce the exposure to X-rays, a helical scan is dividedly executed, and an individual image SD is designated for each scan. This increases the time and labor for imaging. In addition, the X-ray emission area must be set to be larger than the reconstruction range in each scan, and hence this may rather increase the exposure to X-rays.